11

Magnetic Observatory and Repeat Station Measurements in Moçambique Jean L. Rasson, Institut Royal Meteorologique (IRM), Centre de Physique du Globe, B-5670 Dourbes, Belgique Francisco da Silva Sumbana, Manuel F. Kampine, Armindo A. Nhatsave, Direcção Nacional de Geologia (DNG), Dept. de Geofisica Global, Caixa Postal 217, Maputo, Moçambique and John C. Riddick, British Geological Survey (BGS), West Mains Rd., Edinburgh EH9 3LA, Scotland

Abstract This paper will focus on the magnetic observatories of Maputo (LMM) and Nampula (NMP),

presenting the current efforts deployed to operate them and examining the available data. We will also examine some historical repeat station data and the measurements taken at the

stations which were reoccupied during the 2001 field trip.

Introduction The work described here is the result of collaboration between several people and organisations,

all striving towards the same goal, i.e. to get high quality geomagnetic ground observations in the Republic of Moçambique and to establish a durable and sustainable infrastructure for the collection of these data in the long term.

A very useful framework for achieving this is the INTERMAGNET organisation, which has set standards for instrumentation, measurement protocols and data collection and transmission.

When inspecting a map of Sub-Saharan Africa, (Figure 1) where the locations of the various Magnetic Observatories is displayed, it is immediately obvious that there are not many observatories in this region, and that large gaps exist, with many countries having no magnetic observatory infrastructure at all. Sudan, Kenya, Somalia, Tanzania, Malawi, Zimbabwe, Zambia, Botswana, Angola and Democratic Republic of Congo form an enormous contiguous and adjacent group of countries where no magnetic information is systematically gathered. This large geographical void has a negative impact on many practical and scientific activities. It represents a strong hazard to people using a magnetic compass for navigation in those countries.

The distance between Hartebeesthoek (HBK) and Maputo (~500km) is the smallest between two observatories in Africa, and this is fortunate as those observatories cover a part of the continent where the spatial variation of the declination secular variation is the largest.

Nampula Observatory (NMP) was installed in the late seventies and was operative only a few years. We visited this station during this work and took preliminary steps to facilitate its reopening.

Francisco da Silva Sumbana, Manuel F. Kampine and Armindo A. Nhatsave are the observers in charge of Maputo and are responsible for the routine operations in the LMM geomagnetic observatory as well as the field-work.

John Riddick from BGS was the first to assist LMM in 1993 after a difficult period for the Republic and his involvement. This continues to the present day by facilitating equipment and advice to the LMM observatory. The British Geological Survey has been involved with the hand scaling of the LMM magnetograms since the late eighties. Jean Rasson made visits to LMM in 2000 and 2001 with the purpose of giving training in the use of DI-fluxgate’s for observatory and field use and to repair the variometers. He participated in a field trip to Beira and Nampula in July 2001. Dourbes, Niemegk and Hermanus Observatories invited the Mozambican scientists to their IAGA workshops in 1994, 1996 and 2002, and this helped greatly in the training, management and planning of the Observatory operation.

12

Figure 1: View of the Magnetic Observator ies distr ibution in Afr ica and far ther to the South. NMP is not operating and Beira is a repeat station. Orthographic projection centered on Maputo (LMM).

We cannot conclude these introductory notes without acknowledging the huge contribution of the Servicio Meteorologico Nacional (SMN) and other Institutions from Portugal. We appreciate the careful work of many individuals (let us mention Srs A.F.Paulo, F.Pastor, C.Gonçalves, Oliveires Barreiros, Ferraz, Mendes Victor) in planning the Geomagnetic Observatories and the fieldwork in the Mozambican territory. The archives of the DNG are witnesses of the excellent measurements taken by them over the past century.

Recent histor ical perspective Magnetic measurements were first performed in Moçambique and in the adjacent seas by the

early sailors and pilots en route from Europe towards the Indies. Some measurements are available from the seventeenth century.

Routine continuous observations started in 1957 with the building of the Magnetic Observatory in Lourenço Marques, the present Maputo. Below we will give some historical milestones. It is once more clear that successful magnetic observations result from stable political conditions.

1956 Construction of the Magnetic Observatory of Lourenço Marques (Maputo) by the

SMN (Portugal) on the site of the International Airport 1957 Start of magnetic observations in Lourenço Marques

13

1960-1961 First field magnetic measurements at 22 national airports 1964-1966 Production of a magnetic map of Moçambique based on a network of 238 stations.

The reference epoch was 1966.5. Temporary observatories were set-up in Vila Pery and Nampula. Secular variation was computed at the points also measured in 1960/61

1971 Starting the construction of Nampula Observatory. 1982 Start of observations in the Magnetic Observatory of Nampula. 1987 Magnetic Observatory of Nampula is closed. 1989 Stopped the observations in Maputo 1993 Contacts between DNG and BGS in order to restart the magnetic observations. 1994 Srs Sumbana and Matosse, in charge of LMM, participate in the IAGA

Geomagnetic Instrument Workshop in Dourbes. 1996 BGS installs a digital variometer at the observatory of Maputo.

The LMM Magnetic Observatory This Observatory is situated on the premises of the international airport of Maputo, in direct

vicinity of the aircraft taxi and runways. It was built on fenced-off ground of 1 hectare in area, situated inside the closed zone belonging to the airport. This configuration is very satisfactory since it provides a double fencing for the observatory and it is completely encircled by land subject to international law.

The absolute house is a large L-shaped building, equipped with not less than 9 non-magnetic pillars, most having a view on suitable targets, one of the targets being a pillar inside the Observatory grounds. The variometer house (for a description, see the BGS report i) has two spacious vaults and two ground floor laboratories. The first vault is equipped with a large pillar. Vault 2 has two pillars which were built for the classical photographic variometer and recorder set-up. There is a second variometer house, a ground floor construction. Additionally, 3 more buildings are provided as living quarters for staff.

Var iometr ic Instrumentation Since 1996, a digital variometer consisting of an EDA DHZ tri-axial fluxgate, a BGS 16 bit

data logger and an Olivetti laptop computer operated in the first vault of the variometer house. The correct timing is provided by a Motorola Oncore GPS receiver. The EDA sensor has been installed on a short aluminium pole cemented in the concrete floor near the large pillar in the southern extremity of the vault.

It is known that the EDA is not a high precision variometer as it has a high temperature coefficient, but with careful and frequent baseline control and efficient thermal isolation and high thermal inertia, this instrument can give satisfactory results. Further details relating to stability are given later in this paper.

Absolute Instrumentation In August 2000, BGS donated an ELSEC 820 proton magnetometer and a Ruska DIflux besides

a FLM1/B fluxgate electronics to Maputo Observatory. This was a much needed step since the Askania declinometer was out of order since the end of 1999, and the Scintrex Proton Precession Magnetometer MP2 was behaving erratically. Therefore, reliable baseline observations based on sound absolute measurements restarted effectively in August 2000.

The Ruska DIflux was converted from a Ruska nonmagnetic theodolite to a DIflux at Dourbes Observatory Instrumentation Laboratory. The theodolite was a gift from the Dourbes Instrument Pool by the US Geological Survey. To read the Ruska circles two microscopes 180° apart are used, each giving a resolution of 30 seconds of arc. This resolution on the reference mark sighting can be improved to nearly 15 seconds by the observer by taking readings on each side of the circles once with the vertical circle to the left and once to the right (a procedure called "plunging"), and using the mean of these four readings. A Pandect LFG-A13 fluxgate sensor was mounted in a V-groove

14

in the sensor holder mounted on top of the telescope as shown in Figure 2. A 6 m cable connects it to the FLM1/B fluxgate electronics which have a 0.1nT resolution

The theodolite was carefully checked for residual magnetism, and magnetically cleaned where necessary. As a final check of the overall accuracy of the Ruska DIflux, (Serial Number RO1) a series of intercomparison measurements were performed in Dourbes Magnetic Observatory. These measurements were between RO1 and the observatory reference Zeiss 010 (Serial Number 116) DIflux, using both instruments to measure the D0 and I0 baselines of the ELSEC 8200 DIDD variometer. The results of this comparison are given in Table 1. The sensor collimation (ESI, EAZ) and magnetisation (ESO) errors as well as the vertical I gradient (GI) are given.

15

Date D0 ESO ESI EAZ I 0 ESO ESI GI Instr

Units: ° NT " " ° nT " "

13-jul-00 -1.340 -1. -17 73 65.457 3.5 -9 19 RO1

13-jul-00 -1.343 0.6 18 5 65.456 0.7 20 4 116

14-jul-00 -1.338 1.6 -17 85 65.458 2.1 -7 13 RO1

14-jul-00 -1.341 0.8 21 4 65.457 0.3 19 4 116

8-aug-00 -1.338 -1. -19 74 65.458 -3. -8 5 RO1

8-aug-00 -1.342 0.3 19 8 65.456 1.0 22. 4 116

Table 1. Intercompar ison of Ruska RO1 and Zeiss 116 Difluxes by D and I baseline measurements in Dourbes. See text for column header descr iption.

Figure 2. This shows the Ruska theodolite telescope with hor izontal axis / ver tical circle transformed for DI flux operation. The box for mounting the fluxgate sensor is screwed on the ver tical circle, hence rotating with the telescope. Inside the box, in a V-groove, the Pandect fluxgate sensor can be seen.

Subtracting the mean baseline values obtained by the two instruments, we can assess the relative accuracies: D0(RO1-116) = 0.0031° = 11” and I0(RO1-116) = 0.0012°= 4” . As the 116 is known to

16

be free from defects, we may safely conclude that the RO1 has the same quality, since the differences lie beyond the RO1 overall circle reading resolution.

The ELSEC 820 proton magnetometer was checked in the Dourbes Scalar Magnetometer Calibrator and found to be free from errors at the 0.2 nT level. See also the Hermanus 2002 Workshop scalar magnetometer tests in these Proceedings for further intercomparison measurements of this magnetometer.

Observatory Reference Changes During the installation of the new absolute instrumentation (Aug 2000), all parameters of the

observatory were checked and the most convenient pillar was selected as a new reference. This pillar is named “pillar 3” . Figure 3 shows some of the pillars.

Change in F measurement as result of change in instrument from Scintrex MP2 #2 to ELSEC820 and change of pillar X to pillar 3

dF = F3 - Fx = +10.9 nT.

Change in H measurement as result of change in instrument from QHM to DIflux RUSKA RO1 and change of pillar 2 to pillar 3

dH = H3 - H2 = -16.2 nT.

Change in D measurement. Presumed zero.

dD = 0

Table 2: Changes in the Geomagnetic field reference, resulting from moving observations from the old to the new instrumentation and pillars in the observatory.

Figure 3. The Absolute House in the Maputo (LMM) observatory. The pillar in the foreground is pillar 2, used in the past for QHM measurements. In the background, with the Ruska DI flux, is pillar 3, presently the reference pillar for LMM.

17

As a result of this site change, the changeover from the Scintrex proton magnetometer to the ELSEC 820 and the discontinued use of the QHM as absolute instrument for H measurement, LMM observatory experienced a change in the H reference. In order to know the magnitude of this change the pillar differences were measured by using the new absolute instrumentation. The pillar differences are given in Table 2. It is noteworthy to mention that the Scintrex MP2 PPM #2 was calibrated in the Dourbes IAGA workshop in 1994 and had given the calibration equation: F = FScintrex#2 +(-4nT), for the field values in Maputo. This shows that the value of the F jump resulted partly from the correction of a calibration error of the Scintrex.

Figure 4

18

Figure 5

To the best of our knowledge there was no change in the D reference. This is because there was

no pillar change, and the Askania declinometer must have been acceptable, as seen from inspection of the EDA baselines from September 1999 to December 1999. However, we cannot give any guarantee for this as the EDA D channel has been out of order since the beginning of 2000.

First Baseline Results with the Digital Instrumentation In Figure 4 we show the D-channel baseline of the EDA variometer for the period August -

December 2000. It can be appreciated that the stability of this channel is good: the stability is certainly better than 0.01° over the whole period shown. However, the dispersion of the successive measurements is more or less 1 minute of arc. This baseline noise is probably caused by the daily variation of the baseline due to diurnal temperature fluctuations.

In Figure 5, we give the baseline for the horizontal component during the same period. Here also the good stability of the measurements is noticeable, although a baseline shift of about 4 nT occurs near day 300. The dispersion of the successive measurements is less than 2 nT, indicating that the temperature influence on the H-channel is reasonably low.

Figure 6 gives the Z baseline. Here a 12 nT jump starting around day 300 compromises the stability record of the EDA instrument. Similarly, the dispersion of a few nT is similar to the H.

19

Figure 6

An essential routine task at the Observatory is the annual processing of the recorded data of the past year. It is good practice to have some tools to determine the good quality of the data processing. Thus, a final check when preparing these data for the observatory is the comparison of the definitive minute means data with the absolute spot measurements performed weekly for baseline computation. This check is a rather exhaustive one since any difference between the two data sets (minute means – spot measurements) will pinpoint errors in the absolute measurements as well as errors in the processing of the baseline and errors in the minute means. A simple way to compute the absolute spot measurements is to redo the baselines with zero variometer input. This check is illustrated in Figure 7 by investigating the magnetic declination for a section of the month of January 2001.

20

Figure 7. The intercompar ison of the definitive minute means data and the spot absolute measurements taken regular ly for the baseline determinations is a good check of the quality of the data. Any difference between the two sets will indicate errors in the measurement and/or processing of the data.

Difficulties Exper ienced in LMM with Data Acquisition Also noticeable in Figure 7 are two gaps in the definitive data. They are caused by long (>10h)

power failures in the observatory’s electrical supply. Unfortunately this will increase during 2001 and culminate in May 2002, when observations will have to be stopped altogether due to the complete failure of the electrical mains supply. Now (August 2002) repairs to the cable supplying the observatory from the power transformer are programmed for the near future. An alternative solution to this problem would be the complete supply of the Observatory with solar power using photoelectrical panels. The financial estimates have shown that costs would be approximately the same for solar supply as for replacing the power cable. On the other hand, solar supply has a disadvantage, like yearly costs of battery replacements, and possible security problems. Also this would not solve the electrical supply of the rest of the observatory needs like room and premises lighting and living quarter’s commodities.

Secular Var iation in Maputo (LMM)

We give in Table 4 the monthly means for the Observatory of Maputo (LMM) obtained from the digital variometer and new absolute instrumentation. The final months of 2001 are not available for the reasons given in the previous paragraph.

The last published annual mean for LMM before this work began dates back to 1989, meaning a large gap of 11 years before restarting the observatory in 2000. In order to have an indication on the status of the magnetic field during this period, it is interesting to look in Figure 12 at the field in the neighbouring observatories of Antananarivo (TAN) in Madagascar and Hartebeesthoek (HBK) in South-Africa (see Figure 1 for the locations of these observatories). TAN has a very long series of observations dating back to the 19th century and HBK is operating since 1973, nicely covering the period without data in Maputo.

Figure 12 illustrates the spectacular difference in declination secular variation between relatively close Magnetic Observatories in this part of Southern Africa. This problem will be dealt with in greater detail in the paper by P Kotze published in this Proceedings.

21

Repeat Measurements in Beira Thanks to the interest and the financial support of “Aeroportos de Moçambique” (AOM), it was

possible to implement a field trip to Beira and Nampula with the purpose of performing magnetic repeat measurements there. AOM asked us to perform additional True and Magnetic Heading measurements of their runways at the National airports of Beira and Nampula.

For setting-up our field stations, we needed a non-magnetic tripod, which was unobtainable in Maputo Observatory. Finally we located one in a warehouse to the South of Maputo owned by the DNG, where scientific equipment was stored. It is an Askania field galvanometer tripod. Unfortunately it was not possible to modify the length of the legs, so we had to accurately position the tripod level above the mark before mounting and levelling the theodolite. Furthermore, it was necessary to manufacture a small bronze adapter so that the Ruska Diflux could be securely fixed to it. Later we found two adjustable non-magnetic tripods in the NMP observatory.

We perused the archives of the DNG in order to find the exact location of the last repeat station measurements in Beira and Nampula and found nearly all the relevant repeat station information files about both sites. Unfortunately once on site it was impossible to find any markers left by the previous party in 1965. Therefore, we set up our station along the runways in Beira as exactly as possible on the same spot as described in the Beira repeat station file. This was done using the Lat/Long info and a detailed sketch found in the file.

For the benefit of future measurement sessions and field trips, we constructed a small concrete platform with a marker and 3 holes at 120°, into which the theodolite tripod feet should be placed (see Figure 8). In the Appendices of this survey report is the original report (in Portuguese) about this repeat station as well as the results of our repeat observations and runway orientation measurements. The results are reported in Table 3.

Figure 8. The newly constructed repeat station marker platform in Beira. The station is near the runways of Beira airpor t, exactly on the previous (1965) repeat station location. While the observer is measur ing, the masons are enjoying a well earned break.

Examining the reduction of the Beira Estacion Magnetica spot measurements to the LMM baseline (5 last columns in Table 3), it is apparent that the 3 first observations performed at dusk on day 194 are preferred above those performed outside the diurnal variation period. This explains the H baseline jump of 17 nT on day 197, performed shortly before noon. However, in absence of any

22

indication of the difference in field behaviour between Maputo and Beira, results have been computed as the straight mean of all measurements, but excluding the data of day 197. It is also noticeable from the results that the Beira runway is magnetically clean. The final results are given in the Appendix, as item 4.

Figure 9. Measur ing on the runway in Nampula. Note the use of a double tr ipod configuration for supporting the DI flux.

Figure 10. View of the magnetic Observatory of Nampula. In the centre is the outdoor target pillar . Behind it the absolute house and to the extreme r ight the var iometer house.

23

Visiting the Magnetic Observatory of Nampula, Repeat MeasurementsThe region of Nampula is quite spectacular with its many basaltic outcrops, emerging everywhere like strong towers from the ground. The horizon is therefore dotted with those steep rocks, not unlike the “meteors” from the Trikala region in Greece, the “mogotes” in Cuba near Piñar del Rio or even the “Pão de Azucar” in Rio de Janeiro. The outcrops are visible in Figure 9. However those geological conditions have an impact on the surface magnetism as basaltic rocks are magnetic. Previous magnetic measurements in the region have shown strong local anomalies and differences from point to point (up to 1.9° in declination). Therefore we were pleased to have the carefully selected and planned Magnetic Observatory of Nampula (NMP) for our use to process our measurements, even if no measurements had been made there since 1982.

Figure 11. Measur ing in the absolute house of Nampula Magnetic Observatory.

This NMP Observatory is a geophysical station with co-located seismic and geomagnetic facilities. The seismic part is presently actively operating. The Geomagnetic Observatory is equipped with a large absolute house, a variometer house, office space and a sturdy outdoor target pillar (see Figure 10). The conditions in the absolute house were still suitable for performing measurements. Although the windows had disappeared only a little cleaning was necessary before we could start the observations as shown in Figure 11. Excessive vegetation prevented us from observing the target from the D pillar. Therefore we measured D, I and F on the Z pillar, to the West in the absolute house. As in Beira, the observations taken at dusk seem to give a better reduction to the baseline in LMM, which is at a 1400 km distance (Table 3). But here also our computations are based on the straight mean of all measurements.

Realistic magnetic measurements on the runways of Nampula Airport were impossible, due to high gradients and therefore we measured only their true azimuth and computed their magnetic azimuth from the magnetic declination obtained in the nearby Observatory as explained in the Appendix.

Present and Past Data With the availability of the recent measurements, it is worthwhile to try and summarise the

whole geomagnetic field data-set available at the three stations of Maputo, Beira and Nampula. Therefore we visited once more the valuable archives of DNG, looking for an inventory of all the

24

measurements taken at those places over time. We could indeed collect quite a few repeat station measurements; dating back to the late 19th century.

We realize that the quality of the measurements is not homogeneous and that the re-locations from one field measurement to the other can be bad or not cared for altogether. Nevertheless the data give us a rough idea of the secular variation during the whole 20th century at our three stations. The long series taken in Antananarivo gives a guideline to visually interpolate the noisy series (see Figure 12).

Figure 12. An attempt to summarise the available Magnetic Observatory data set for the Eastern par t of Southern Afr ica. This is a mix of Observatory annual means and spot/repeat station measurements. Note the strong difference in secular var iation for stations as close as HBK and LMM.

Compar isons between LMM and HBK for Rapid Var iations Since the digital data collected in LMM has been processed to get final results for the

INTERMAGNET * .bin minute means format, it is quite easy to compare the minute data with neighbouring INTERMAGNET observatories, HBK for instance. As an example, daily plots for October 31th 2000 are given in Figure 13 and Figure 14. The similarity between both plots is striking, except for the Z channel. The Z rapid variation in HBK seems to be completely suppressed.

Conclusions To conclude this work, it is important to point out once more that the observatory of Maputo is

still at risk of being closed due to electrical mains disruption. The observatory of Nampula remains closed. We strongly recommend that the future of Maputo Observatory be guaranteed in the long term and that the observatory operations resume in Nampula.

We believe that this paper has shown the presence of both those observatories due to: 1. The excellent Magnetic Observatory infrastructure available in the Republic of

Mozambique, complemented by the donated modern equipment, 2. The high level of expertise available both for observatory operations and field work, 3. The interest shown by the international scientific community for magnetic observations in a

region with extreme secular variation gradients, 4. The strategic location of Nampula in a region with a shortage of magnetic observatories, 5. The interest shown by the commercial partners in Mozambique and

25

6. The possibility to export the Mozambican know-how to neighbouring countries.

Acknowledgements We appreciated very much the contribution from “Aeroportos de Moçambique”, in the form of

logistics, travel, financial support and most important, interest shown for the work described in this paper. The efficient help provided by their employees Sr Marcos Paulino, Sr Geraldo Sangue in Beira and Sr. Lopes in Nampula was very valuable.

INTERMAGNET paid the travel expenses for Jean Rasson for his flight from Belgium to Moçambique in 2001.

Figure 13

26

Figure 14

DINAGECA in Maputo kindly put its cartographic expertise at our disposal for clarifying some geographic coordinate issues.

We thank Dr Dias Joao in Nampula for helping with long term planning. Dr Dani Ceuninck smoothed our way by judiciously flashing his diplomatic passport when

appropriate. We are indebted to Lee Pankratz from the US Geological Survey for donating the excellent

Ruska Theodolite which would become the RO1 DIflux. Finally we would like to thank our Parent Institutes (BGS, DNG and IRM) and their Directors

who authorised those activities and contributed in no small way to the success of this project.

27

Appendix

Field report: Levantamentos de dados magnéticos nos Aeroportos de Maputo, Beira e Nampula em julho de 2001

1.- Descr ição Geral

Chegado `a Beira `as 10 horas do dia 14 de Julho de 2001,uma brigada geofísica da Direcção Nacional de Geologia constituida pelos técnicos Francisco da Silva Sumbana, Manuel Farnela Kampine, Armindo Alberto Nhatsave e chefiada pelo tecnico Belga Dr. Jean L. Rasson Presidente do Grupo V-I de Trabalhos da IAGA (Associação Internacional de Geomagnetismo e Aeronomia) e de Instrumentos de Precisão, para vir dar continuidade dos trabalhos de treinamentos aos técnicos do Observatório Magnético do Maputo e bem como para efectuar levantamentos magnéticos da Declinação para a calibração em voo da informação radiada pelas estações e DVOR-DME e determinacao de Azimute magnetico nas pistas dos Aeroportos Internacionais de Maputo, Beira e Nampula para a melhor seguranca aerea dos aeronaves nas pistas dos Aeroportos Internacionais de Maputo, Beira e Nampula pedido das Empresas dos Aeroportos de Mocambique.

Chegado ao Aeroporto Internacional da Beira a brigada esteve acompanhado pelo tecnico das Empresas dos Aeroportos de Mocambique vindo de Maputo tambem o Sr.Marcos Paulino e o Sr. Geraldo Sangue tecnico electronico do Aeroporto da Beira, imediatamente a brigada dirigiu-se para o local a procura do marco da Estacao Magnetica, tendo sido feito buscas na pista principal nr. 12 a 75 metros do centro do cruzamento da pista TAXI (ver figura em anexo) e entrado em direccao ao Nordeste perpendicular a pista a 87,5 metros conforme a indicacao das informacoes anteriores de 1962 em mao e dado o facto de nao podermos encontrar o lugar exacto utilizado anteriormente, foi estabelecido a Estacao no lugar precisamente previsto medido por uma fita de medicao de 30 metros. Havendo a hipotese de ter sido retirado pelo tractor nos trabalhos de rotina de limpesa das pistas.

Pelo que foi necessario preparar no proprio lugar previsto uma plataforma da Estacao Magnetica da Beira, isto e foi feita um marco constituido por pedra, areia e cimento de uma forma triangular com um buraco no centro (introduzido um tubo de aluminio e nas tres extremidades tambem com tubos de aluminios para se poder assentar o tripe para observacoes no local feito este trabalho em 16 de Julho de 2001. Nas tres paredes do triangulo foi introduzido tres tabuas de madeira em contraplacado e cuidadosamente nivelado no seu assentamento. Dentro do betao foi introduzido tubos e alguns fios de bronze somente para a sua fortificacao.

Neste trabalho todo estiveram envolvidos os Srs. Alberto Malaidge e Manuel Cueija Joaquim “Well” ambos pedeiros, Dr. Jean L.Rasson, Francisco da Silva Sumbana, Manuel Farnela Kampine, Armindo Alberto Nhatsave, Marcos Paulino, Geraldo Sangue e alguns Bombeiros um dos quais o Sr. Angelo Miguel que trouxeram agua para os mesmos trabalhos .

Gradiante vertical na plataforma(ver figura) da Estacao Magnetica do Aeroporto da Beira com media de 2,6 nT.

Chegada a Nampula em 17 de Julho de 2001 as 15:15h vindo da Beira imediatamento foi solicitado um carro e cedido prontamente pelo Sr. Lopes da Delegacao da Empresa dos Aeroportos de Mocambique em Nampula, procurou-se a pista 05 para poder localizar o marco da Estacao Magnetica de Nampula e nao foi possivel localiza-la tendo se decidido seguir para o Observatorio Magnetico de Nampula.

As casas do Observatorio Magnetico de Nampula estao sem portas, janelas e tecto falso mantendo ainda a cobertura.

Na casa Absoluta foram encontados pilares e decidimos utilizar o pilar mais a Oeste para observacao do campo magnetico. Naquele pilar esta rodeado de tres janelas e tinhamos em vista o pilar da mira exterior que tinhamos identificado anteriormente. Constatou-se que o lugar estava sujo porque as pessoas defecavam, tendo sido necessario criar condicoes para se poder fazer observacoes magneticas dentro da mesma nos pilares, ja era noite seguiu-se para o Hotel para se pernoitar.

28

Nos dias 18, 19 e 20 de Julho de corrente ano continuou-se com os mesmos trabalhos de levantamentos magneticos no Observatorio Magnetico e nas pistas 05 e 32 respectivamente para a determinacao de Azimute e Declinacao Magnetica.

No dia 19 de Julho uma parte da brigada foi recebida pelo Sr. Director Provincial do MIREME em Nampula Branquinho Ferro Nhombe, Sertorio Azevedo M. Aurelio e Dias Joao tendo sido debrucado sobre a Estacao Sismografica e bem como a reabilitacao do Observatorio Magnetico de Nampula para poder receber novo equipamento doado pelo British Geological Survey do Reino Unido.

A Brigada regressou a Maputo no dia 20 de Julho de 2001 e em seguida ja em Maputo a brigada foi a Dinageca para confirmar as coordenadas geograficas dos pontos medidos durante acampanha magnetica, pelo que muito se agradece a boa colaboracao dada pelos tecnicos desta Instituicao.

29

Table 3. Results of our repeat observations and runway orientation measurements. The last five columns give the reduction of the LMM baselines. Estacao Data 2001 Lat Long GrV Mira (Nome) Azimuth Azim Mag D. média de

(DOY) nT gr.d. gr.d. Julho 2001

Beira Aeroporto 194.5653 19° 47m 36s S 34° 54’ 24” E 2.6 mira 2 lado 299.762 -11.288

Est Mag 194.5944 esquerdo

194.6208 torre C.

195.3472

195.4035

197.3465

Beira Aeroporto 195.2910 19° 47’ 37” S 34° 54’ 12” E -1.0 Linha central 104.931 116.242 -11.288

Pista 12 195.4278 da Pista 12

195.4431

195.4576

Nampula OBS NMP 198.3069 15° 05’ 15” S 39° 15’ 16” E pilar externo 166.773 -7.636

pilar 001 (Z) 198.6076

198.6271

Nampula Aeroporto 200.2667 15° 06’ 46” S 39° 16’ 30” E linha central 46.048 53.666 -7.636

Pista 05 200.2931 da pista 05

200.3042

Nampula Aeroporto 198.4049 15° 06’ 01” S 39° 17’ 21” E linha central 226.037 233.668 -7.636

Pista 23 da pista 23

198.4340

Maputo Aeroporto -189.6972 25° 55’ 29" S 32° 34’13” E -6.5 linha central 28.890 46.675 -17.699

Pista 05 -189.7076 da pista 05

-189.7014

Maputo Observatorio 194-198 25° 54' 59" S 32° 34' 47" E lado direito do 235.757 -17.699

LMM edi.torr contr

30

Table3 (continued) Estacao D H [nT] Z[nT] F[nT] I D0 H0 Z0 F0 I0

gr.d. gr.d.

Beira Aeroporto -11.202 16439.4 -25561.0 30391.1 -57.253 -11.765 16403.3 -25566.0 30375.7 -57.315

Est Mag -11.218 16435.3 -25563.8 30391.2 -57.263 -11.760 16399.6 -25568.1 30375.6 -57.324

-11.253 16432.0 -25566.6 30391.8 -57.271 -11.764 16393.9 -25571.6 30375.4 -57.336

-11.440 -11.771

-11.356 -11.746

-11.321 16475.9 -25542.0 30394.8 -57.176 -11.726 16411.4 -25559.0 30374.3 -57.296

Beira Aeroporto -11.344 -11.747

Pista 12 -11.326 -11.751

-11.293 -11.745

-11.279 -11.740

Nampula OBS NMP -7.676 20034.0 -23881.8 31172.1 -50.007 -8.113 19981.6 -23919.3 31167.3 -50.126

pilar 001 (Z) -7.587 19997.1 -23895.0 31158.5 -50.075 -8.098 19957.7 -23900.5 31137.5 -50.137

-7.614 19999.9 -23892.3 31158.2 -50.068 -8.101 19955.6 -23900.3 31136.0 -50.140

Nampula Aeroporto -8.596 -9.035

Pista 05 -8.623 -9.021

-8.652 -9.028

Nampula Aeroporto -8.300 -8.702

Pista 23

-8.239 -8.668

Maputo Aeroporto -19.044 -19.427

Pista 05 -19.067 -19.463

-19.035 -19.443

Maputo Observatorio -18.167 13928.0 -25865.0 29377.0 -61.698

LMM

31

Table 4. Monthly means in LMM with the digital var iometer and new absolute instrumentation

Date [Years] D [°] I [°] F [nT] H [nT] X [nT] Y [nT] Z [nT] 2000.62 -17.5557 -61.6606 29405.2 13958.5 13308 -4210 -25881 2000.71 -17.5675 -61.6732 29392.2 13946.6 13296 -4209 -25873 2000.79 -17.5784 -61.6728 29396.4 13948.8 13297 -4213 -25876 2000.87 -17.6069 -61.6567 29397.2 13956.4 13303 -4222 -25873 2000.96 -17.6091 -61.6249 29402.9 13973.5 13319 -4227 -25870 2001.04 -17.6264 -61.6182 29401.9 13976.0 13320 -4232 -25868 2001.12 -17.6294 -61.6078 29400.1 13979.9 13323 -4234 -25864 2001.20 -17.6508 -61.6327 29392.5 13965.0 13308 -4234 -25863 2001.29 -17.6752 -61.6639 29386.8 13948.2 13290 -4235 -25866 2001.37 -17.6793 -61.6149 29393.4 13973.5 13314 -4244 -25860 2001.45 -17.6852 -61.6000 29393.7 13980.4 13320 -4247 -25856 2001.54 -17.6992 -61.5882 29394.4 13986.0 13324 -4252 -25854 2001.62 -17.7144 -61.5866 29390.7 13985.0 13322 -4255 -25850 2001.71 -17.7391 -61.5561 29394.4 14000.5 13335 -4266 -25846

No dia 24 de Julho a brigada trabalhou na pista principal 05 de Maputo. A pesar de vento forte e baixa temperatura fizemos observações de limpeza magnética com Magnetometer de Protões na pista 05 concluimos que a pista tinha maior Gradiente no campo magnético. Fizemos várias observações Astronómicas do Sol e magnéticas de Declinação. O resultado está na tabela. A diferença de 0.6 graus entre o Observatório de Maputo e a pista 05 confirma a falta de limpeza magnética da pista 05 do Aeroporto de Maputo por isso utilizamos a declinação medida no Observatório Magnético de Maputo para calcular o Azimute Magnético da pista 05.

Na tabela também damos a declinação média mensal de Julho do ano 2001. Tem se a salientar a boa colabaracao dispensada na Beira,Nampula e Maputo pelos tecnicos

que colaboraram para a boa execucao dos trabalhos desta Brigada Geofisica.

2.- Metodologia 1. Tratou-se de encontrar o marco deixada pela brigada anterior se existia. Fez-se de duas

maneiras : perguntar as pessoas do mesmo lugar ou utilizar a informacao deixada pela brigada anterior. Se a marca nao fosse encontrada instalar a estacao no lugar identico segundo a informacao disponivel.

2. Averiguar a limpeza magnetica do lugar utilizando o magnetometro de protoes fazendo-se dois perfiles ortogonais intersectando pelo lugar da estacao. O perfil tem dez metros e com uma medicao em cada metro e com a distancia de 25 cm do solo . Adicionalmente fazendo a medicao de gradiante vertical no lugar da estacao: diferenca entre o campo total medido no solo com o campo medido em cima do tripe.

3. Depois de identificar as miras e conhecendo as coordenadas geograficas da estacao fazer a observacao do Sol para determiar os azimutes das respectivas miras. Deve ser utilizado um relogio em tempo universal com precisao de um segundo.Para a reducao das observacoes utiliza-se o software DOS Soleil.exe e o anuario Astronomico do periodo correspondente.

4. Fazer as observacoes absolutas da Declinacao e Inclinacao magnetica utilizando o Diflux fixado em cima do tripe.

5. Calcular o azimute magnetico da pista se necessario utilizando uma das duas formulas: Azimute magnetico = Azimute - Declinacao Azimute magnetico = Leitura da mira - leitura meridiano magnetico 6. Fazer a observacao absoluta da forca total utilizando o magnetometro de protoes fixado em

cima do tripe na mesma altura que o eixo horizontal do Diflux. 7. Reduzir as observacoes de D, I e F calculando o campo magnetico instantaneo. Calcular a

linha de base do variometro do observatorio mais proximocom as observacoes absolutas da estacao. Calcular diferencas com a linha base do observatorio calculada com a medicao absoluta do observatorio.

3.- Detalhe dos resultados

Damos abaixo uma tabela com todos os resultados obtidos. Apesar de não poder localizar os marcos das estações magnéticas chegamos a uma precisão de relocalização de uns metros na

32

Beira. No aeroporto de Nampula encontramos dificuldades devido a anomalias magnéticas. Como indica a diferença de 0.3 graus na Declinação Magnética entre as extremidades da pista principal. Também tivemos dificuldades na medição naquela pista com Magnetometer de protões, o que indica o importante gradiente no campo magnético ambiente. Nos arquivos de DNG encontamos o mapa que indica também fortes gradientes (até 1.9 graus) na região de Nampula. Por esse motivo utilizamos a Declinação medida no Observatório Magnético de Nampula especialmente preparado para esta ocasião para calcular o Azimutes Magnéticos da pista principal de Nampula

4.- Diferenças entre o Observatór io Magnético de Maputo e as Estações visitadas Utilizando os valores médios das linhas de bases da tabela obtivemos as seguintes equações: a) Estação Magnética do Aeroporto da Beira

DBeira[graus] = DLMM + (- 11.755 + 18.167)=DLMM + 6.412 HBeira[nT] = HLMM + (16402-13928) = HLMM + 2474 ZBeira[nT] = ZLMM + (-25566.2 +25865) = ZLMM + 298.8 FBeira[nT] = FLMM + (30375.2 -29377) = FLMM + 998.2 IBeira[graus] = ILMM + (- 57.318 + 61.698)=ILMM + 4.380

b) Observatór io Magnético de Nampula DNampula[graus] = DLMM + (-8.104 + 18.167)=DLMM + 10.063 HNampula[nT] = HLMM + (19965.0-13928) = HLMM + 6037 ZNampula[nT] = ZLMM + (-23906.7 +25865) = ZLMM + 1958.3 FNampula[nT] = FLMM + (31146.9 -29377) = FLMM + 1769.9 INampula[graus] = ILMM + (- 50.134 + 61.698)=ILMM + 11.564

5. – Recomendações Para melhor segurança Aérea dos Aeronaves é necessária uma cobertura nacional pelos mapas

magnéticos de Moçambique. Para obter esta carta é necessário reocupar várias Estações Magnéticas no interior do País e sobretudo as Estações fronteiriças. Estas últimas são importantes para evitar efectos de bordo durante a interpulação e extrapulação de mapas à actualizar. Por isso recomendamos a reocupação das Estações de Zumbo, Tete, Pemba, Lichinga e Quelimane. Essas Estações seriam o mínimo para poder chegar matematicamente a um mapa magnético actual a partir do mapa de 1975 como o último desponível actualmente. As Estações fronteiriças nacionais também permitiriam a D.N.G. prestar serviços e produtos magnéticos aos Paises visinhos